Squelch tail circuits are known. Squelch tail circuits are used in simplex radios, such as those typically in service with police or fire departments, to mute the audio portion of a receiver following reception of a communicated signal.
Simplex radios are typically constructed to transmit upon activation of a push-to-talk (PTT) button and receive upon release of the PTT button. For an operator to receive a communicated signal, the simplex radio must detect the incoming signal and unmute the audio portion of the receiver.
The signal detection function in simplex radios is typically based upon signal strength measurements. If a detected signal is above a threshold then an un-mute signal is sent to the receiver un-muting the audio portion of the receiver. At the end of the communicated signal the loss of signal must be detected and the receiver again muted (squelched) to avoid annoying bursts of noise.
While the threshold method of muting works well, problems arise under weak signal conditions. Under weak signal conditions and if the threshold is set too high, fluctuations in detected signal level may result in the radio muting prematurely. If the radio mutes prematurely, before the end of the transmission, portions of the communicated signal will be lost. If, on the other hand, the threshold is set too low then, upon termination of the signal, the radio may not mute.
The solution to the problem of weak signal muting has in the past been provided by a variable time delay on muting after signal loss. The variable time delay is important because as the signal becomes weaker the percentage of time that the signal exceeds a threshold may decrease. A variable time delay may allow a very weak signal to fade in an out due to signal fluctuations without prematurely muting.
The variable time delay on muting a simplex radio is provided by a squelch tail circuit. A squelch tail circuit varies the time delay on muting, depending on detected signal strength. A squelch tail circuit upon detecting a strong signal may mute immediately after loss of signal. Upon detecting a weak signal the squelch tail circuit may provide a relatively long time delay before muting since loss of signal may represent fluctuations in received signal.
A squelch tail circuit typically measures the magnitude of out-of-voice-band noise present on an audio channel (as a measure of signal strength) and, based upon the detected level of noise, adjusts a time delay for activation of the mute function. For signals with a very low level of detected out-of-band noise (strong signals) the delay is very short. For signals with a very high level of out-of-band noise (weak signals) the delay is relatively long.
Shown in FIG. 1 is a schematic of a prior art squelch tail circuit. Used as an input for the squelch circuit is a Squelch Noise Input signal comprised of rectified out-of-voice-voice-band signal components from a limiter (not shown). The rectified out-of-voice-band signal can be assumed to be near 0 v for a very strong signal (significant quieting) and may be as high as 2.5 v for a weak signal (very little quieting).
The squelch tail circuit is enabled (the audio portion of the simplex radio is un-muted) by the presence of a relatively low level signal at the Squelch Noise Input terminal. Enablement of the circuit occurs through activation of the hysteresis comparator. The hysteresis comparator is activated by the presence of a relatively small DC voltage presented at the minus (-) terminal of the hysteresis comparator (indicating the presence of a carrier). Activation of the hysteresis comparator, in turn, activates the current enable and changes the output voltage of the squelch detect voltage reference.
The current enable enables the current follower. Enablement of the current follower causes the output of the current follower to output a current proportional to the voltage difference between the Squelch Noise Input signal and the capacitor causing the capacitor to charge to a value substantially equal to the Squelch Noise Input signal.
Deactivation of the current follower causes the current follower to present a substantially open-circuit condition to the resistor-capacitor (RC) network connected to the output of the current follower and the plus (+) input to the squelch detect comparator. Deactivation of the current follower allows the capacitor to discharge through the resistor. Discharge of the capacitor causes the capacitor voltage to eventually equal a first threshold voltage found at the output of the squelch detect voltage reference. As the plus (+) input to the squelch detect comparator drops below the minus (-) input to the squelch detect output comparator the output of the squelch detect comparator reverses causing the squelch detect circuit to provide a mute signal to the receiver of the radio circuit.
In operation a strong signal presented to the radio would present a relatively low voltage signal to the Squelch Noise Input and, in turn, charge the capacitor to a relatively low voltage. At the end of a transmission the DC signal at the Squelch Noise Input would rise to a high level causing the hysteresis comparator to disable the follower comparator (presenting an open circuit by the current follower to the RC network). Since the capacitor was charged to a relatively low level the output of squelch detect comparator would quickly shift to a muted state.
In the case of a weak signal presented to the radio a relatively large DC voltage would be present at the Squelch Noise Input terminal and the capacitor would charge to the relatively high level. At the end of the transmission the hysteresis comparator would detect the end of transmission and disable the current follower. Since the capacitor is charged to a relatively high level a relatively long capacitor discharge time must now pass before the capacitor voltage equals the first threshold. If the hysteresis comparator were to re-detect a signal then the current follower would again be enabled and the capacitor would recharge.
As described above use of the squelch tail circuit may provide a relatively long period from loss-of-signal to mute under weak signal conditions. The relatively long time period is beneficial in the control of simplex radios under weak signal conditions.
While the squelch tail circuit has provided reliable performance the squelch tail circuit does not lend itself to miniaturization or to modern manufacturing practice. The need for relatively long squelch tail periods requires the use of a relatively large capacitor (2.2 microFarads). Furthermore, time constants relying on RC networks exhibit poor consistency due to manufacturing variations inherent in on-chip resistors. A smaller capacitor, offering comparable performance, requires a larger resistor. Large capacitors or resistors are difficult to construct using integrated circuit construction techniques. A need exists for a way of constructing a squelch tail circuit offering comparable performance yet compatible with integrated circuit manufacturing capabilities.